I had bought 4 neon tetras today from petco, and they were in a tank with a betta. I put them in the tank about 3 hours ago or so, and 1 died and another one is super lathargic and "laid" on the rocks. Whats going on?
I did put Bettafix in the tank before, is that what is killing them?

It would be helpful if you posted the tank parameters and setup before we try and give you any advice.

Size?
Heated/filtered?
Ammonia/nitrite/nitrate/pH levels?
How long it has been running
Any other symptoms on any fish?
Where you got the new fish

Bettafix: I don't recommend it for bettas because the main ingredient Melaluca (sp?) is damaging to their labrynth organ. However, it shouldn't have an effect on the tetras though unless they weren't properly acclimated. BTW, how did you acclimate them?

I took a sample of my water into Petco and took the fish I could find that had died back, and I guess my ammonia levels were too high. And I found out that neon tetras weren't the best idea to add to a newer tank.

I had the tank up for about 2 weeks, the lady at petco said it wasn't cycled enough.. When I introduced them in the tank I did what my old man always did and kept them in the back but put the bag in the water for about 30 minutes.

I float my guys for ten minutes, open the bag and pour one cup of my aquarium water in the bag and float for ten more minutes, add one more cup and float for five more minutes. Then I pour the contents of the bag into a bucket through a net and catch the fish and place it in the tank. I try to do that as quickly as possible. I also turn off all the light in my room.

What exactly were the water parameters? How high was the ammonia? What are your pH and temperature? Normally, a slight increase in ammonia is still not toxic enough if your pH is on the acidic to neutral side whereas temperature is no higher than 80 degrees. Alkaline water and high temperature make ammonia much more toxic as both are elevated further. If you do not have a test kit for yourself yet, I'd invest in API liquid drops. If not, at least make sure the store is using liquid test kits that are not expired or tampered with. Have them write the results as well.

BakaMandy is correct. If you don't have a quarantine tank, I'd invest in one at least 10g. Smaller tanks are much more difficult to stabilize the water conditions which is why a 10g is much better investment. Besides, tanks smaller than a 10g are considered novelty items and it is ironic they are much more expensive than a 10g. Quarantine every new fish you get for at least 3-4 weeks.

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